A sub-tropical storm with hurricane-strength winds is taking aim at New Zealand. The weather will pack up today before the storm makes landfall tomorrow.

The storm is on track to hit the top of the country tomorrow before sweeping down the country, affecting all the North Island and the north and east of the South Island.

Heavy rain is expected to drench Northland and Auckland tomorrow before spreading across the island. Potentially destructive winds are expected to barrel across the North Island for days.

Waves up to 7m high are expected to hammer all North Island coastlines.

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MetService says heavy rain warnings are likely for Coromandel Peninsula, Bay of Plenty and the western ranges of Gisborne when the storm hits late Thursday.

The country's forecaster is warning the significant weather system has the potential to bring severe weather to northern and central New Zealand.

Weatherwatch.co.nz head analyst Philip Duncan said the storm would rapidly deepen and intensify before it made landfall tomorrow.

Campers and trampers were warned to be aware of the deteriorating conditions. The storm was expected to bring torrential rain, widespread strong to gale-force winds and dangerous sea and beach conditions around the North Island.

Duncan said models showed the storm was expected to peak early Friday. A wind map put the worst of the winds becoming hurricane force - 120 km/h - for a time at the centre but such winds would stay mainly out at sea.

The North Island is in line for more thunderstorms today when daytime heat will lead to large cloud build-ups and isolated torrential downpours with thunderstorms.

Bay of Plenty and Gisborne, western North Island areas from Waitomo to northern Manawatu are at greatest risk along with regions at either end of the South Island.

MetService says any thunderstorms that develop will be slow-moving and have the potential to produce localised heavy rain or downpours.

The rain could be so intense people are being warned to expect up to 40mm in one hour in western and central North Island.

The forecaster says this level of drenching could lead to flash flooding and make driving conditions extremely dangerous.

Duncan said the approaching storm was likely to cause very dangerous marine conditions, bringing strong rips and mountainous seas affecting western and eastern coastlines of the North Island.

He said those near streams should be prepared to move to higher ground should the low produce flooding rains, which is possible in some spots - especially after such a long dry end to spring and start to summer.